There are many different types of sponsors available for you to choose from. Some are really easy to get in, and some are really tough to get your website accepted. The top players are Google, Yahoo, ValueClickMedia, BurstMedia and CasaleMedia. If you are accepted by all 5 companies, you might want to quit your job when the money starts to come in :p. Anyways here is a brief introduction to each company and how they pay you for running the ads.

Google
Google is one of the most popular search engines in the world, and matter of fact they have been on number one list for a very long time. Google's program is called Google Adsense. The concept is you run Google's Sponsors ads on your website. Then if someone clicks those ads on your website, you get certain amount per click. For example an advertiser could bid $1.00 for his ad, means if someone clicks it, you will get $1.00 . Its extremely easy to sign-up, just go here and register.
http://adwords.google.com/

Yahoo
Yahoo is fairly new in this but rapidly made a good reputation. Yahoo works exactly the same way but from my experiences Yahoo pays more than google, so I might well think about signing up with Yahoo atleast if not Google.
http://publisher.yahoo.com/

ValueClickMedia
This company is my favorite, because their payment ratio is great and I don't need to wait for people to click on ads. Honestly when you visit some website do you click on Google or Yahoo ads? Let me guess very rarely... so same with your website, there's no garuantee that people will click on your ads. This is why ValueClickMedia is my favorite, because they don't pay you for each click but for the amount of times their ad is shown. This system is called CPM, means Cost Per Thousand. M is the roman number for 1000, so you will get certain amount for each 1000 views. ValueClickMedia usually pays at least $5.00 per 1000 views. If you notice that you aren't getting much clicks on Google or Yahoo then its time for you to switch to just CPM system.
http://www.valueclickmedia.com/

BurstMedia
BurstMedia works the same way as ValeClickMedia but bit tough to be accepted. Their payment ratio is awesome and sometimes even higher than ValeClickMedia, so you really should sign-up with this company as well.
http://www.burstmedia.com/

CasaleMedia
Same as BurstMedia and ValueClickMedia, but their payment ratio is little less than both companies. You can sign-up for this company if you wish to earn some few extra bucks.
http://www.casalemedia.com/

Before making a layout for your arcade website, we highly recommend you to think of the proper places where your ads will be. Please NEVER NEVER and I mean NEVER put your advertisement all the way top of your layout and/or all the way bottom. People will just ignore those and the CPM companies won't pay for the ads displayed in those places. The reason why they won't is because obviously you are just trying to place the ads and make money, and not trying to lure your visitor to that website. But on the other hand your sponsor wants the visitor to see the ad, so if you are not supporting their primary goal, they won't pay you. In addition don't use 468 x 60 ads, unless you must. It is recommended to place a Skyscraper (120 x 600) advertisement on the left or right side of your arcade website. And make sure the ads are integrated within your layout. Here are some good arcade website for you to see and learn the proper ad placement techniques.

http://www.freearcade.com/
http://www.miniclip.com/
http://www.gametap.com/
http://www.realarcade.com/